In many applications using thin film structures (e.g. integrated circuits (ICs), active matrix liquid crystal displays (AMLCDs) and the like), low resistivity of the metal lines in those structures is important for high performance. Low resistivity in metal lines minimizes RC delay which translates into, for example, faster screen refresh rates for AMLCDs. Refractory metals, such as chromium (Cr), molybdenum (Mo), tantalum (Ta), and tungsten (W), have too high a resistance for use in high performance displays. Additionally, the cost of refractory metals as standard metal lines is greater than other non-refractory metals. From the standpoint of low resistance and cost, aluminum is a desirable line metal.
Aluminum, however, has an unfortunate tendency to form defects, called "hillocks", under certain deposition conditions. These defects are characterized by protrusions that form on the side of the aluminum that is parallel to and away from the substrate. Hillocks are often times fatal to the correct operation of the IC or active matrix because the protrusions may "punch" through several layers overlying the aluminum. For these reasons, many attempts have been made to suppress the formation of hillocks during the fabrication process.
The first of these techniques employs a separate metal (such as tantalum) capping layer over the aluminum metal. This technique has been regarded as successful for controlling the formation of hillocks in the fabrication process. One drawback, however, is that a separate masking fabrication step is needed to pattern the capping metal--otherwise the capping metal layer, extending over the entire plate, is more likely to cause a short. This is in contrast to a dielectric capping layer which need not be patterned to avoid such an electrical short. This additional masking step for the capping metal adds to the overall cost of fabrication and production.
Another drawback to this technique is the greater line width of the resultant capped structure. In order for capping to successfully avoid the formation of hillocks, the capping layer must be exactly abutting the aluminum on all sides. If the registration of the capping metal is less than accurately placed, then hillocks have been known to form. At the present error tolerances of photolithography, the additional thickness of the capping metal consumes a sizable percentage of the total line width. Because the capping metal is generally not as good a conductor as aluminum, the resistivity of the capped metal line is greater than for a line of aluminum of the same width.
A second technique caps metal with an oxide layer by means of an anodic process. One such attempt is described in the article: "P-6: Low-Resistivity Tantalum Film for TFT Gate Line" by Shimada et al., published in SID 93 Digest at page 467. Shimada et al. describe the anodic growth of a tantalum oxide layer on tantalum which is additionally capped by a layer of SiN.sub.x.
The same anodic process has been used to grow a layer of aluminum oxide on aluminum. The problem with such oxides is that they are generally regarded as poor insulators as well as poor suppressors of hillock formation. It has been reported that small defects in the oxide layer ("pinholes") may form, through which hillocks may protrude. Another drawback with the use of anodic oxide is that the anodic process generally requires the use of potentially contaminating electrolytes and also requires a means of connecting all the patterned lines to achieve anodic growth on all the lines simultaneously.
Additionally, neither of the above methods for controlling hillocks simultaneously addresses the concern of fabricating low resistive metal layers. To address both concerns simultaneously, the factors (e.g. the temperature of later fabrication steps, the thickness of the metal layer, and the like) contributing to the formation of hillocks must be considered. Essentially, metal, such as aluminum, is deposited by physical vapor deposition (i.e. evaporation and sputtering). The temperature of the substrate during aluminum deposition may be varied according to the desired properties of the aluminum. For example, at higher deposition temperatures, the resistivity of aluminum decreases and its grain size increases.
Patterning of the aluminum is done at room temperature by standard lithographic process. After patterning, a dielectric layer, such as an SiN layer, is deposited at temperatures in the range of 300-380 degrees C. This high temperature is generally necessary to achieve good dielectric properties. This reheating causes stress in the aluminum metal because of a mismatch in the coefficients of thermal expansion between the substrate (usually glass in an active matrix liquid crystal displays) and aluminum. The additional stress is relieved in the form of hillocks, whose growth can measure in size from tenths of a micron to several microns.
Apart from temperature, the growth of hillocks is also dependent upon the thickness of the deposited metal layer. As a general rule, the thicker the layer, the larger the strain energy of the film, and the more likely is the possibility that hillocks will form at a given temperature. Thus, it is possible to reduce the amount of hillock formation at a given temperature by reducing the thickness of the metal layer in question. It is well known that, for a given temperature, a "critical" thickness exists--below which hillocks are unlikely to form.
Reducing the thickness of the metal layer below the critical thickness, however, has one major drawback--comparatively thin metal layers have comparatively high resistivity. For the purposes of high performance thin film structures, thin metal layers are generally not acceptable.
Thus, there is a need for the suppression of the hillocks in metal layers without reducing the resistivity of the metal layer.
It is an object of the present invention to create metal layers that are substantially hillock-free while simultaneously of low resistivity.